A necessary step in the production of solvent refined coal (SRC) and other coal liquefaction products is a solvent fractionation or distillation step. Such fractionation is usually accomplished in a distillation tower at atmospheric or below atmospheric pressures. The liquids to be fractionated contain a number of constituent species. Nitrogen compounds, most of which can be classified as amines, constitute a significant fraction of these liquids. Oxygenated compounds of the phenol variety and chloride from the coal dissolved in the process solvent are also present. The chloride is generally associated with the nitrogen compounds, and forms the amine hydrochloride within the tower. It has been found that the chemical makeup of the coal liquids within the fractionation tower are highly corrosive. Corrosion rates on the order of 1,000 mpy (mils per year) have been reported for stainless steel tower materials. The severity of this corrosion has required the relining, retraying, or total replacement of fractionation apparatus.
It has been determined that the corrosive constituents in coal liquids are phenols, sulfur compounds, and certain water soluble chlorides. These three constituents interact synergistically to corrode iron based alloys in a reaction with an approximate ultimate stoichiometry of: EQU Fe+2HCl-&gt;FeCl.sub.2 +H.sub.2 ( 1)
Because of the stoichiometry indicated by equation (1), limitation of chloride concentrations will necessarily limit the amount of iron which can be corroded by a given amount of coal liquid. (If the reaction were catalytic rather than chemical, each chloride ion could be responsible for the dissolution of many metal atoms.)